Perfluorosulfonate ionomers (PFSI's) such as E. I. duPont de Nemours & Co.'s Nafion polymer, are proving to be versatile membrane materials in a variety of applications. For example, PFSI's have been used in electrochemical processes and devices including chloralkali cells, batteries, fuel cells, and water electrolyzers. However, many applications require PFSI's to be formed into films. This is frequently accomplished through dissolving the PFSI in a solvent, depositing the solution onto a substrate, and removing the solvent. High-temperature, high-pressure procedures for dissolving PFSI membranes has recently been reported in U.S. Pat. No. 4,433,082 and in an article by C. R. Martin; T. A. Rhoades, and J. A. Ferguson, Anal. Chem., 1982, 54, 1639. Using such procedures, solutions of PFSI have been formed and used to prepare PFSI-coated electrodes, catalysts, and high performance liquid chromatography packing materials. However, such methods do not produce a film that is entirely satisfactory because its properties are different from the polymer before it was formed into a film.
Any film-forming method used with PFSI's preferably should produce PFSI films which possess the solubility behavior, chemical, mechanical, and thermal stability equivalent to the PFSI material as it was originally produced. For example, films formed by prior art methods, such as those discussed above, are easily dissolved in an ethanol/water mixture, while the original polymer was not so easily dissolved. In addition, the PFSI's cluster-channel morphology is changed when the polymer is formed into a film. The cluster-channel morphology is important because it helps to impede anion transport, which is a desirable property if these films are accomplish the task for which they are being used, i.e., to reject anions. The present invention provides a method which can be used to produce films having properties similar to those of the polymer before they were formed into the film.
References which teach dissolving a PFSI in a high boiling point solvent include: "Electrochemical, IR and XPS Study of Nafion Films Prepared From Hexamethylphosphortriamide Solution", Journal of Electroanalytical Chemistry, Volume 199 (1986) pages 81-82; "Study of Nafion Films On Electrodes Prepared From Dimethylacetamide Solution", Journal of Electroanalytical Chemistry, Volume 200 (1986) pages 379-381; and U.S. Pat. No. 4,433,082, "Process For Making Liquid Composition of Perfluorinated Ion Exchange Polymer, and Product Thereof", W. G. Grot.